If you want to convert a face or vertex normal, normal
, from local space to world space, you do it like so:
var normalMatrix = new THREE.Matrix3(); // create once and reuse
var worldNormal = new THREE.Vector3(); // create once and reuse
...
normalMatrix.getNormalMatrix( object.matrixWorld );
worldNormal.copy( normal ).applyMatrix3( normalMatrix ).normalize();
three.js r.107